This invention relates to a stagnation prevention system for use in an information processing system of the type comprising a plurality of widely dispersed information processing stations, with singular or plural information processing devices, connected in cascade connection through information transmission lines, with information transmission being conducted mutually between optional stations, and the transmission between adjoining stations being respectively independent.
Since information processing stations are usually installed fairly distant from each other, connecting all the stations individually requires a large amount of money for transmission lines and a large amount of labor to install the transmission lines. To eliminate such a disadvantage in information processing systems of the type described, the plural information processing stations are connected in series, in a closed or open loop, as shown by the plural stations S.sub.1 -S.sub.n in FIG. 1. The stations are shown connected in order through the information transmission lines L.sub.1 -L.sub.n.
In such an information processing system, the construction is such that one information transmission is not executed until the transmitted information from one station circulates through all the stations and returns to the station where the information is emitted, in order to attain a priority processing function and, further, to check for errors in the transmitted information. In addition, each station is provided wth a bypass means which is switched into operation when the station is not operating irrespective of the cause of the inoperativeness. This prevents the failure of a single station from shutting down the entire system.
However, if the information from one station is required to circulate through all the stations and return to the source station, as mentioned above, several disadvantages result. The time for a single information transmission is relatively long and the efficiency of information transmission is low.
To eliminate the latter problem, an information processor of the construction as shown in FIG. 2 has been proposed.
In FIG. 2, Si (i = 1 - 5) are information processing stations widely dispersed. Each of the stations is provided with various information processing devices Dij (i = 1 - 5, j = 1 - n) which are connected through the common inner information transmission bus ILi (i = 1 - 5). The inner transmission buses are controlled by the bus control devices BCi (i = 1 - 5). Also, each station Si is equipped with two communication devices Tij (i = 1 - 5, j = 1, 2) having bilateral communication capabilities and being connected to the internal bus ILi. Each communication device, Tij, of each station is connected to one of the communication devices of an adjacent station via one of the external information transmission lines, OLi (i = 1 - 5). For example, the communication device T.sub.12 of station S.sub.1 is connected to the communication device T.sub.21 of station S.sub.2, and the communicating device T.sub.22 of station S.sub.2 is connected to the communication device T.sub.31 of station S.sub.3, etc., so that the five stations S.sub.1 -S.sub.5 are connected in series in a closed loop, constituting an information processor of the closed loop type.
Therefore, in an information processor constructed in this manner, the communication devices installed at each station have the following functions: to receive information from the internal bus of the station to which they belong, which information is addressed for transmission to an information processing device in another external station, and to transmit that information to an external transmission line connected thereto; and to receive information from the external transmission line connected thereto and transmit that information to the internal bus of the station to which they belong. The transmission of information between the communication devices and the processing devices in a station is subject to the control of the bus control device in the same manner that information is transmitted between the processing devices. However, the transmission of information to and from other external stations is free from control by the bus control device. Such information is freely and independently conducted between communication devices which are connected via an external line.
The transmission of information within a station, to and from the communication devices, is controlled by the bus control device as follows. As an example, assume that information addressed to processing device D.sub.3j in the station S.sub.3 is transmitted to the internal bus IL.sub.1 in the station S.sub.1 from the information processing device D.sub.1j or the communication device T.sub.1j in station S.sub.1. When the system is working properly, the bus control device BC.sub.1 will control the bus IL.sub.1 so as to cause the information to circulate in a prescribed direction. If the direction is the right hand direction, the control is such that the transmitted information is put into the communication device T.sub.12. When the communication device T.sub.12 thus receives the information, destined for another external station, it transmits said information via the external transmission line OL.sub.1 to the communication device T.sub.21 of station S.sub.2. Now, assuming that the communication device T.sub.11 receives transmitted information from communication device T.sub.52 of station S.sub.5, via external transmission line OL.sub.5, device T.sub.11 emits the information received to the internal bus IL.sub.1 with permission of and under the control of the bus control device BC.sub.1. In this case, if the information is addressed to an information processing device in the station S.sub.11, the bus control device BC.sub.1 will control the bus IL.sub.1 to cause the information to be put into the information processing device D.sub.1j to which the information is addressed. If, on the other hand, the information is addressed to an information processing device in a station other than S.sub.1, the bus control device BC.sub.1 will control the bus IL.sub.1 such that the information will be put into the other communication device T.sub.12 ; similarly to the case when such information is emitted from an internal information processing device. Thus the information will be transmitted outside the station S.sub.1 through the communication device T.sub.12 without being put in the station S.sub.1 and passes through the station S.sub.1.
All of the other stations work similarly to S.sub.1. Thus, if the information is to be transmitted from S.sub.1 to S.sub.3, the information generated in station S.sub.1 is first transmitted by communication device T.sub.12 to comunication device T.sub.21 of station S.sub.2 via external transmission line OL.sub.1. Then, in the station S.sub.2, the information received by communication device T.sub.21 is sent to communication device T.sub.22, since it is not addressed to the station S.sub.2, and is transmitted to communication device T.sub.31 of adjoining station S.sub.3. In station S.sub.3, the information received by the communication device T.sub.31 is sent to bus IL.sub.3 from communication device T.sub.31, since it is addressed to one of the information processing devices T.sub.3j inside station S.sub.3 itself, and is transmitted to the designated information processing device D.sub.3j. Thus, the information is not transmitted outward, thereafter. Similarly, information transmission may be executed between any two stations.
In the kind of information processor described, where information transmission is conducted freely and independently between adjoining stations, the transmission efficiency of information is remarkably improved due to the capability of simultaneous information transmission between all adjoining stations.
In this kind of information processor, however, when an error arises in the transmitted information received by the station S.sub.2 because of the mixture of noise from the transmission line OL.sub.1, etc. in the transmission of information from the station S.sub.1 to S.sub.2, transmission of the same information is repeated from the stations S.sub.1 to S.sub.2 until correct information transmission is attained. During that time, therefore, new information can not be transmitted from the station S.sub.1 to S.sub.2, so that new information which is generated in station S.sub.1 and addressed to other stations stagnates and is stored temporarily in a buffer register of a buffer circuit installed in the communication device T.sub.12. When the buffer circuit becomes full, the stagnation of the information transmission in station S.sub.1 spreads to the preceding station S.sub.5, causing the buffer circuits in the station S.sub.5 to fill up. Similarly, stagnation of the information transmission spreads to the adjoining stations in order and, in the worst case, the buffer circuits for external transmission in all the stations become full, terminating all information transmission between the stations and bringing the whole device to a deadlock.